1. Field of the Invention
The present invention relates to a novel zoom lens, and particularly to a zoom lens suitable for a camera, such as a video camera and a digital still camera, which receives an incident light by an image pick up element.
2. Description of Related Art
As recording means in a camera, there is known a conventional method of converting and recording a quantity of light per image pick up element of an object image formed on the surface of the image pick up element to an electrical output by an image pick up element using an photo-electric conversion element such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal-Oxide Semiconductor).
With the progress of fine processing technology in recent years, high-speed of a central processing unit (CPU) and high packing density of recording media have been made, thus realizing to process huge volumes of imaging data at high speed hitherto beyond processing. Further, high packing density and miniaturization have been made with regard to the light-receiving element, so that the high packing density made it possible to record in higher spatial frequencies, and miniaturization made it possible to make a camera small as a whole.
However, through the above-mentioned high packing density and miniaturization, there was a problem in which a light-receiving surface area of each photo-electric conversion element became narrow, thus increasing noise as in electric output decreases. To prevent this, the quantity of light to reach over the light-receiving element was increased by enlarging the aperture ratio of the optical system, and a minute lens element (so-called micro lens array) was provided immediately before each element. The above-mentioned micro lens array, instead of guiding the optical flux reaching between adjacent elements on to the elements, position of an injection pupil of the lens system was restricted. Namely, it is because, as an angle formed by a main light ray reaching the light-receiving element relative to the optical axis becomes large, a off-axis flux of light towards a periphery of a image plane makes a large angle relative to the optical axis, resulting in not reaching over the light-receiving element and causing a shortage of the quantity of light.
Various proposals have been made regarding inventions of zoom lenses suitable for cameras which record the image of an object through these photo-electric conversion elements.
As a zoom lens for the video camera, there is in the main stream a so-called positive-negative-positive-positive 4-group zoom lens consisting of four lens groups of a positive lens group, a negative lens group, a positive lens group, and a positive group in that order from the object side. Especially, in regard to variable magnification, there used to be in the main stream a zoom type in which the first lens group and the third lens group were set relative to an optical axis direction, the second lens group functioned as a variator, and the fourth lens group functioned as a compensator. Specifically, for example, a zoom lens listed in Patent Document 1 is known.
With higher packing density of recent light-receiving elements, the lens system has been made compact while its performance has improved. Especially, to provide for a compact and higher performance system, it is essential to correct properly fluctuations of various aberrations which accompany changes in the state of lens positional status (from the wide angle-end condition to the telescopic end condition).
In the positive-negative-positive-positive 4-group zoom lens, there is only one lens group having negative refraction, thus creating a problem of difficulty to correct negative distortion-aberration in the wide-angle end condition. Especially, since the variator belonged to the second lens group, it was difficult to weaken refraction of the second lens group because of necessity to obtain a preset ratio of variable magnification, so that it was necessary to correct the negative distortion-aberration through other lens groups. Consequently, the third lens group used to include a group of a positive part (part having positive refraction) and a group of a negative part (part having negative refraction) such that negative distortion-aberration which tended to occur in the wide-angle end condition could be properly corrected. At the same time, since the third lens group converged the flux of light diverged by the second lens group, it had strong positive refraction. For the above-mentioned reasons, the structure of the third lens group was important.
Specifically, for example, in a zoom lens described in Patent Document 2, the third lens group consisted of a convex lens, a convex lens, and a concave lens. In a zoom lens described in Patent Document 3, by constituting the third lens group with a cemented lens of a biconvex lens, a convex lens, and a concave lens, it was so designed to suppress performance deterioration due to mutual decentering generating inside the third lens group at the time of manufacture.
Further, in a zoom lens described in Patent Document 4 , the third lens group consisted of a convex lens and a concave lens, while the fourth lens group consisted of a positive lens.
[Patent Document 1]
Japanese Published Patent No. Sho62-206516
[Patent Document 2]
Japanese Published Patent No. Hei6-308388
[Patent Document 3]
Japanese Published Patent No. Hei9-281392
[Patent Document 4]
Japanese Published Patent No. Hei5-107473
[Patent Document 5]
Japanese Published Patent No. Hei4-361214
[Patent Document 6]
Japanese Published Patent No. Hei4-43311